The invention relates to a method for controlling the brake pressure in at least one wheel brake mounted on a vehicle axle during a braking operation on a road surface having a heterogeneous coefficient of friction.
The invention also relates to a device for controlling the brake pressure difference between the brake pressure in a wheel brake on the low coefficient of friction side and the brake pressure in a wheel brake on the high coefficient of friction side of the vehicle with a heterogeneous coefficient of friction.
When braking on a heterogeneous road surface having different coefficients of friction on the right and left longitudinal sides of the vehicle, asymmetrical braking forces may occur, resulting in a yawing torque which induces a rotational movement of the vehicle about its vertical axis. To prevent skidding of the vehicle, the driver must build up a compensatory yawing torque for suitable steering movements to counteract the torque produced by the asymmetrical braking forces. Locking of the wheels should be avoided here, even on the side of the vehicle having the low coefficient of friction, because the great reduction in transmissible lateral traction of a wheel associated with such locking can prevent a buildup of the required compensatory torque.
In vehicles having an antilock brake system (ABS), locking of the wheels is prevented by a controller. In situations of the aforementioned type, a control strategy conventionally used pursues the goal of decelerating the vehicle with a short braking distance by means of the highest possible brake pressure in the wheel brakes on the side of the vehicle having the higher coefficient of friction. On the other hand, the driver should not be overstressed due to a yawing torque caused by different braking forces on the side with the low coefficient of friction versus the side with the high coefficient of friction (driver response time in countersteering).
Therefore, in the situations in question, the brakes are controlled on the basis of a so-called yawing torque buildup delay (GMA) so that a difference between the brake pressures on the high and low coefficient of friction sides is built up only slowly on the front axle of the vehicle in order to allow the driver enough time to execute stabilizing steering maneuvers. In the wheel brakes on the rear axle, the brake pressure is limited to the value allowed for the low coefficient of friction side (“select low”) so that enough lateral traction can be built up on the rear axle to be able to stabilize the vehicle through steering intervention measures.
The measures described here allow the driver a simpler means of controlling the vehicle, but the potential coefficient of friction on the high coefficient of friction side is not optimally utilized for decelerating the vehicle.
Unexamined German Patent DE 197 07 106 A1 describes how the brake pressure in the wheel brake on the wheel on the high coefficient of friction side, the so-called high wheel, may be controlled on the basis of a quantity representing the driving status of the vehicle, said quantity being formed as a function of the deviation between the yaw rate of the vehicle and a nominal yaw rate calculated from the steering angle set by the operator of the vehicle. Thus, instead of the select-low control method described above, a brake pressure control method that is individual for each wheel is used.
If the vehicle yaws in the direction of the high coefficient of friction side, a pressure buildup on the high wheel, however, leads to an intensification of the yawing motion of the vehicle. The known method thus has the disadvantage that it could destabilize the driving status of the vehicle in possible driving situations.
Therefore the object of the present invention is to improve upon the braking performance of a vehicle in braking on a heterogeneous road surface and at the same time to reliably ensure the driving stability of the vehicle.